1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which stores “1”- and “0”-data using a magnetoresistive effect.
2. Description of the Related Art
In recent years, many memories which store data by new principles have been proposed. One of them is a magnetic random access memory which stores “1”- and “0”-data using a tunneling magnetoresistive. (to be referred to as TMR hereinafter) effect.
As a proposal for a magnetic random access memory, for example, Roy Scheuerlein et al, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, p. 128 is known.
A magnetic random access memory stores “1”- and “0”-data using TMR elements. As the basic structure of a TMR element, an insulating layer (tunneling barrier) is sandwiched between two magnetic layers (ferromagnetic layers). Note that various TMR element structures have been proposed for the optimization of an MR (MagnetoResistive) ratio.
Data stored in the TMR element is determined on the basis of whether the magnetizing states of the two magnetic layers are parallel or antiparallel. “Parallel” means that the two magnetic layers have the same magnetizing direction. “Antiparallel” means that the two magnetic layers have opposite magnetizing directions.
Normally, one (fixed layer) of the two magnetic layers has an antiferromagnetic layer. The antiferro-magnetic layer serves as a member for fixing the magnetizing direction of the fixed layer. In fact, data (“1” or “0”) stored in the TMR element is determined by the magnetizing direction of the other (free layer) of the two magnetic layers.
When the magnetizing states in the TMR element are parallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the TMR element is minimized. For example, this state is defined as a “1”-state. When the magnetizing states in the TMR element are antiparallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the TMR element is maximized. For example, this state is defined as a “0”-state.
Write/read operation principles for TMR elements will be briefly described next.
TMR elements are arranged at the intersections of write word lines and bit lines. Write operation is performed by supplying currents to a write word line and a bit line and determining the magnetizing direction of the free layer of a TMR element using the magnetic field formed by the currents flowing in the two lines.
For example, in write operation, a current flowing only in one direction is supplied to a write word line, and a current flowing in one or the other direction is supplied to a write bit line in accordance with write data. When a current flowing in one direction is supplied to a write bit line, the magnetizing state of the TMR element placed at the intersection of the write word line and the write bit line becomes parallel (“1”-state). When a current flowing in the other direction is supplied to a write bit line, the magnetizing state of the TMR element placed at the intersection of the write word line and the write bit line becomes antiparallel (“0”-state).
Read operation is performed by supplying a read current to a selected TMR element and detecting the resistance value of the selected TMR element.
The read operation principle greatly changes depending on the array structure of a magnetic random access memory. In an array structure in which one switching element is connected in series with one TMR element, the switching element connected to a selected read word line is turned on to supply a read current to a selected TMR element. This read current is guided to a sense amplifier to read the resistance value of the selected TMR element, thereby determining the data in the TMR element.
There is one big problem with read operation.
Since a read current passes through the insulating layer (tunnel barrier) in a TMR element, the resistance value of the TMR element greatly depends on the thickness of the insulating layer. More specifically, the resistance value of the TMR element logarithmically changes with changes in the thickness of the insulating layer in the TMR element.
More specifically, the thickness of a tunnel barrier in a TMR element currently reported is about several nm. However, as variations in tunnel barrier thickness increase among a plurality of TMR elements, variations in resistance value logarithmically increase.
It is therefore difficult for a magnetic random access memory to adopt a sense scheme using reference cells like those used in a NOR type flash memory.
Assume that in a magnetic random access memory, the resistance value of a selected TMR element is compared with that of a reference cell by using a differential sense amplifier to read the data stored in the selected TMR element. In this case, the data must be prevented from being buried in noise due to variations in tunnel barrier thickness.
That is, a resistance change (the difference between the resistance value in a parallel magnetizing state and that in an antiparallel magnetizing state) ΔR determined by the MR ratio (magnetoresistive change ratio) must be sufficiently increased relative to variations in the resistance values of TMR elements and reference cells.
However, the currently feasible MR ratio is 20 to 40% in general, and about 50% at maximum. At such MR ratios, in consideration of manufacturing margin and yield in mass production, it is impossible to attain the resistance change ΔR of a TMR element large enough to prevent data from being buried in noise.
As a proposal for solving the above problem associated with read operation, a technique of storing 1-bit data in two TMR elements is known. More specifically, in this technique, correct data is stored in one of two TMR elements, opposite data is stored in the other TMR element, and the two data are compared at the time of a read. According to this technique, the resistance change ΔR based on the MR ratio can be substantially increased twice.
In this case, however, storage of 1-bit data in two TMR elements disadvantageous increases the memory capacity. In addition, this technique cannot completely eliminate the influence of variations in resistance value among a plurality of TMR elements. Depending on the magnitude of variations in resistance value among TMR elements, therefore, the resistance change ΔR of a TMR element may not be sufficiently large.
A breakthrough technique of solving the problem associated with a read, i.e., the problem associated with variations in resistance value among a plurality of TMR elements, and offering an advantage in increasing the integration degree of memory cells and increasing memory capacity has been disclosed in U.S. Ser. No. 09/961,326.
A magnetic random access memory using this technique has an array structure having a plurality of TMR elements connected in parallel with each other.
In read operation, a read current is supplied to a plurality of TMR elements connected in parallel, and the resistance value of the plurality of TMR elements is detected by a sense amplifier. Thereafter, predetermined data is written in a selected one of the plurality of TMR elements. After this operation, a read current is supplied to the plurality of TMR elements connected in parallel again, and the resistance value of the plurality of TMR elements is detected by the sense amplifier.
The resistance value of the plurality of TMR elements, detected previously, is compared with the resistance value of the plurality of TMR elements, detected afterward. If they are substantially equal, it is determined that the data in the selected TMR element is the predetermined data. If they differ from each other, it is determined that the data in the selected TMR element is data having an opposite value to that of the predetermined data.
According to this read operation principle, the resistance value (or MR ratio) of a selected TMR element can be accurately read regardless of variations in resistance value among a plurality of TMR elements in read operation. The feasibility of a magnetic random access memory using this read operation principle is therefore high.
In this read operation principle, the data stored in a selected TMR element may be destroyed (destructive read) at the time of read operation. After the data value in the selected TMR element is determined, therefore, data must be rewritten in the TMR element.
Although the technique disclosed in U.S. Ser. No. 09/961,326 is very effective as described above, no specific proposals of a write circuit such as a write driver and a read circuit such as a sense amplifier have been made. In addition, the array structure of a magnetic random access memory and the read operation principle need to be further improved to increase the feasibility.